1. Field of the Invention
The present invention relates to stack type batteries used for, for example, robots, electric vehicles, and backup power sources, more particularly to stack type lithium ion batteries that can improve charge-discharge characteristics at high rate.
2. Description of Related Art
In recent years, batteries have been used for not only the power source of mobile information terminal devices such as mobile telephones, notebook computers, and PDAs but also for such applications as robots, electric vehicles, and backup power sources. This has led to a demand for higher capacity batteries. Because of their high energy density and high capacity, lithium secondary batteries are widely utilized as the power sources for such applications as described above.
The battery configurations of lithium ion secondary batteries are broadly grouped into two types: cylindrical type, in which a spirally wound electrode assembly is enclosed in a closed-end cylindrical battery case, and stack type, in which a stacked electrode assembly comprising plurality of stacks of rectangular-shaped electrodes is enclosed in a closed-end prismatic battery case or a battery case prepared by welding two laminate films together or welding the peripheral portions of a folded laminate film together.
Of the above-described lithium ion secondary batteries, the latter type employs a stacked electrode assembly, in which a required number of sheet-shaped positive electrode plates each having a positive electrode current collector tab and a required number of sheet-shaped negative electrode plates each having a negative electrode current collector tab are stacked with a separator interposed between each of the electrodes. The plurality of positive electrode tabs are overlapped with each other and ultrasonic welded to a positive electrode current collector terminal, while the plurality of negative electrode tabs are likewise overlapped with each other and are ultrasonic welded to a negative electrode current collector terminal (see Japanese Published Unexamined Patent Application Nos. 2003-282044 and 2006-324093).
As described above, the capacity of lithium ion secondary batteries has been increasing in recent years. Accordingly, the number of the positive and negative electrode plates stacked has tended to increase. In addition, the thickness of the positive and negative electrode current collector terminals tends to be made greater, taking into consideration that the battery is charged and discharged at high rate. Consequently, a large number of positive/negative electrode current collector tabs overlapped with each other need to be ultrasonic welded to a thick metal plate (current collector terminal). However, weldability of the weld points tends to be poorer because there is a thickness difference between the positive/negative electrode current collector terminal and the positive/negative electrode current collector tabs and also because a large number of current collector tabs need to be welded simultaneously. This results in variations in the connection resistance values between the positive/negative electrode plates and the positive/negative electrode current collector terminal, so the current values passing through the electrode plates become non-uniform especially when the battery is charged/discharged at high rate (specifically, a large current flows through a portion in which the connection resistance value between the positive/negative electrode plates and the positive/negative terminal is small, while a small current flows through a portion in which the connection resistance value between the positive/negative electrode plates and the positive/negative electrode current collector terminal is large). As a consequence, uneven distribution of the charge-discharge conditions occurs in the battery. This causes partial overdischarge or overcharge in the battery, leading to poor battery cycle performance.
The present invention has been accomplished in view of the foregoing problems, and it is an object of the invention to provide a stack type battery that can minimize deterioration in battery cycle performance even when charge-discharge operations are performed at high rate, by preventing variations in connection resistance values between the positive/negative electrode plates and the positive/negative electrode current collector terminals.